Metal halide discharge lamp having expanded section arc tube

ABSTRACT

The arc tube of a metal halide arc discharge lamp has electrodes at each end thereof and contains a fill including mercury and metal halide. The arc tube has an expanded diameter section intermediate the electrodes, instead of being of uniform diameter.

United States Patent [191 Fohl [ METAL HALIDE DISCHARGE LAMP HAVING EXPANDED SECTION ARC TUBE [75] Inventor: Timothy Fohl, Carlisle, Mass.

[73] Assignee: GTE Sylvania Incorporated,

Danvers, Mass.

221 Filed: July 19,1973

2| App]. N0.: 380,737

[52] US. Cl. 313/220; 313/225; 313/228 [51] Int. Cl. IIOIj 17/16 [58] Field of Search 313/25, 220, 225, 227, 313/228 [56] References Cited UNITED STATES PATENTS 3,543,076 11/1970 Haslund ..313/184 [451 July 22,1975

3,590,307 6/1971 Dobrusskin et a1 313/225 X 3,662,203 5/1972 Kuhl et al. 313/220 X FOREIGN PATENTS OR APPLICATIONS 508,525 12/1954 Canada 313/225 Primary Examiner-James B. Mullins Attorney, Agent, or Firm-James Theodosopoulos [57] ABSTRACT The are tube of a metal halide arc discharge lamp has electrodes at each end thereof and contains a fill including mercury and metal halide. The are tube has an expanded diameter section intermediate the electrodes, instead of being of uniform diameter.

2 Claims, 1 Drawing Figure METAL HALIDE DISCHARGE LAMP HAVING EXPANDED SECTION ARC TUBE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to wall stabilized metal halide arc discharge lamps of the type used for general illumination. Such lamps have a generally cylindrical arc tube having electrodes at each end thereof. The arc tube contains a fill of mercury, metal halide and, for starting purposes, an inert gas. During normal operation, the pressure within the arc tube is between about I to 10 atmospheres and the temperature of the arc tube is between about 500 to l,000C. The are discharge does not extend to the walls of the arc tube and is affected by convection currents within the arc tube.

2. Description of the Prior Art Metal halide arc discharge lamps for general illumination have become commercially useful in the past to years because they are more efficient and yield a whiter light than high pressure mercury vapor lamps. The are tube in such metal halide lamps is made of fused quartz (a high silica glass) and is a straight cylindrical tube having press seals at each end. All presently available commercial metal halide arc discharge lamps for general illumination have a substantially uniform diameter are tube, that is, an arc tube with constant cross section.

Some types of prior art compact source are discharge lamps have been made with non-uniform diameter are tubes, such as short arc lamps and heavily loaded capillary lamps. Such lamps are used with optical equipment and are not usually general illumination lamps. However, in such lamps the arc discharge is not affected by convection currents within the arc tube. For example, short arc lamps generally contain a spherical arc tube and have arc discharges that are electrode stabilized. This means that the arc length is small compared to the arc tube diameter, that the shape of the arc discharge is independent of the shape of the arc tube and that the arc discharge is not affected by convection currents within the arc tube.

Some types of capillary arc discharge lamps have been made with a sllight bulge at the hottest portion of the arc tube in order to prevent melting of the glass thereat. But in such lamps the arc discharge extends to the walls of the arc tube and is confined thereby; thus the arc discharge is constricted and is not significantly affected by convection currents within the arc tube. In addition, capillary lamps are so heavily loaded (watts/sq.cm.) that, in general, they must be artificially cooled in order to prevent the arc tube from melting.

In contrast to compact source arc discharge lamps, the arc discharge in general illumination metal halide lamps is affected by convection currents within the arc tube during lamp operation.

SUMMARY OF THE INVENTION I have discovered that a substantial unexpected increase in efficiency (lumens/watt) of general illumination metal halide arc discharge lamps can be obtained by a change in the shape of the arc tube from the uniform diameter tube that is commonly used. In my invention the arc tube has an expanded diameter section intermediate the electrodes in order to control the flow of convective currents within the arc tube.

ln lamps of the type with which this invention is concerned, there are opposing convective flows within the arc tube of the gaseous and vaporized material therein. When the arc tube is operated with its axis vertical, or at angles other than horizontal, the upward convective flow is essentially along the axis of the arc tube, which is also the axis of the core of the arc discharge. The downward convective flow is near the walls of the arc tube. When the upward and downward flows are in close proximity, the shear between them causes radial convective flows, which transport material between the upward flow and the downward flow. This effect causes the sections of the are which are above the level of the condensate to contain less condensate vapor than the lower parts of the are which are in the vicinity of the condensate. This is called species segregation and is common in conventional arc tubes of the lamps to which this invention relates. The condensate consists of metal halides which are only partially vaporized during lamp operation, such as sodium halide, scandium halide, and the like. The mercury, of course, is entirely vaporized during lamp operation.

Species segregation has several disadvantages in metal halide lamps. Non-uniform species concentration causes non-uniform spectral emission distribution along the axis of the arc. Segregation also makes it difficult to arrive at an optimum concentration of species in the arc, since to provide sufficient species density in the upper arc requires a superabundance in the lower arc.

The purpose of the present invention is to reduce the shear between the upward flow and the downward flow by expanding the diameter of the arc tube at a section intermediate the electrodes. This increases the distance between the upward flow and the downward flow; it also decreases the velocity of the downward convective flow. Reduction of the shear reduces radial mixing between the upward flow and the downward flow; thus the concentration of condensable species can be made substantially uniform along the length of the arc and is then determined by the vapor pressure of the species at the temperature of the condensate.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE in the drawing is an elevational view of a metal halide arc discharge lamp having an expanded section are tube in accordance with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in the drawing, an arc discharge lamp in accordance with this invention comprises an outer glass envelope or jacket 1. Jacket 1 is provided at its end with a sealed reentrant stem 2 through which extend relatively stifi' lead-in wires 3 and 4 connected at their outer ends to the electrical contacts of the usual base 5. Disposed within jacket 1 is an expanded section are tube 6.

Arc tube 6 is supported within jacket 1 by means of metal frames 7 and 8 at each end of arc tube 6. Metal frames 7 and 8 comprise rigid wires 9 and 10, respec tively, to which are fastened clamps 11 and 12 each of which supports a pressed seal end of arc tube 6. Metal frame 7 is supported by lead-in wire 4 to which it is welded. Metal frame 8 is supported at the other end by metal leaf springs 13 which press against the inner wall of envelope 1.

Electrical connection from lead-in wire 4 and metal frame 7 to the proximate main solid electrode 14 is through connective wire 15. Electrical connection from lead-in wire 3 to the other main electrode 16 is through wires 17, 18 and 19. Electrical connection from lead-in wire 3 to the starter electrode 20 is through resistor 21. Bimetal switch 22 shorts starter electrode 20 to the adjacent main electrode 14 after lamp ignition occurs. The atmosphere within jacket 1 is an inert gas, such as nitrogen.

Arc tube 6 has an expanded section at about or near its center in order to substantially eliminate radial convective flow between the upward flow and the downward flow. The result of this is a significant increase in lamp efficiency and substantial elimination of species segregation in the arc.

In an example of a 1,000-watt lamp in accordance with this invention, an arc tube was blow molded into the desired expanded section shape from an openended fused-quartz tube, 18 mm l.D. by 21.6 mm CD. by 6 inches long. The outside diameter of the molded tube was as follows: 0.851 inches for a distance of 1 inch from the upper end, then gradually increasing to a maximum of 1.194 inches for a distance of threeeighths inch starting at a point 2 /2 inches from the upper end, then gradually decreasing to 0.851 inches for the lower seven-eighths inch. After the molded tube was formed into arc tube 6 by embedding the electrodes in the press seals at each end and by adding an arc tube fill of mercury, sodium iodide, scandium iodide and inert gas and then sealing, the ratio of maximum arc tube diameter to minimum arc tube diameter was about 1.4. The arc length (distance between main electrodes) was 91 mm and the ratio of arc length to maximum inside tube diameter was about 3.4. The maximum diameter of arc tube 6 was slightly above the center of the arc tube.

The efficiency of this lamp was 126.5 lumens per watt, which is about 26 percent greater than the efficiency of present commercially available 1,000-watt lamps, which have an efficiency of about 100 lumens per watt. Also, radial convective flow between the upward and downward flows was almost completely eliminated in this lamp and the convective flow pattern could be considered a single convective cell extending almost the entire length of the arc. The flow pattern can be made visible by introducing fine carbon particles into the arc tube and by observing their motion during lamp operation, the carbon particles being heated to incandescence by the arc. If desired, convective velocities can be readily measured by filming the motion of the particles.

The extent of species segregation can be determined by photographing the arc through a filter that passes only light of a wavelength characteristic of a particular metal. For example, to determine the uniformity of the sodium concentration in the arc, a filter that passes the sodium doublet lines at 5900 Angstroms could be used; for mercury, a filter that passes 5770-5790 Angstroms could be used, and for scandium, 4730 Angstroms.

Comparison of commercially available 1,000-watt lamps with the above 1,000-watt lamp shows considerably more uniformity of the concentrations of sodium and scandium throughout the arc in the lamp of this in vention. In addition, the expanded section are tube reduces to a considerable extent the tendency of the mercury to radiate strongly from the upper section of the arc, as occurs in conventional lamps.

In the determination of the optimum diameter for the expanded section of an arc tube in accordance with this invention, are tubes were tested whose maximum diameter varied from about one-fifth the arc length to about equal the arc length. It was found that if the maximum diameter is too small, radial flow is not eliminated; if the maximum diameter is too great, then the result is an unsteady or wavering arc. In general, an increase in the pressure within the arc tube would require an increase in the diameter of the expanded section as would, also, an increase in the density of filling material per unit length of arc length.

The maintenance of lamps in accordance with this invention appears to be about the same or slightly better than that of prior art lamps, that is to say, lamps having uniform diameter are tubes. In one test, 23 1,000-watt lamps were made having an expanded section are tube as per this invention. The average initial efficiency of these 23 lamps was 1 l9 lumens per watt. The average efficiency after 1,000 hours operation was 1 12 lumens per watt, a maintenance of 94 percent. This is about equal to the maintenance of prior art 1,000- watt lamps after 1,000 hours operation, which had an average initial efficiency of lumens per watt and a 1,000 hour efficiency of 94 lumens per watt.

I claim:

1. A general illumination wall stabilized metal halide arc discharge lamp comprising: an outer envelope; a generally tubular arc tube, having electrodes at each end, disposed within said envelope, said are tube containing a fill including mercury, metal halide and an inert gas, the diameter of said are tube being at a minimum at about the electrode regions thereof and gradually increasing therefrom to a maximum diameter at about the center of said are tube; and means to energize said arc tube.

2. A general illumination wall stabilized metal halide arc discharge lamp comprising: an outer envelope; a generally tubular arc tube, having electrodes at each end, disposed within said envelope, said are tube containing a fill including mercury, metal halide and an inert gas, the diameter of said arc tube being at a minimum at about the electrode regions thereof and gradually increasing therefrom to a maximum diameter at a region near the center of said arc tube; and a means to energize said are tube.

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1. A GENERAL ILLUMINAATION WALL STABILIZED METAL HALIDE ARC DISCHARGE LAMP COMPRISING: AN OUTER ENVELOPE, A GENERALLY TUBULAR ARC TUBE, HAVING ELECTRODES AT EACH END, DISPOSED WITHIN SAID ENVELOPE, SAID ARC TUBE CONTAINING A FILL INCLUDING MERCURY, METAL HALIDE AND AN INERT GAS, THE DIAMETER OF SAID ARC TUBE BEING AT A MINIMUM AT ABOUT THE ELECTRODE REGIONS
 2. A general illumination wall stabilized metal halide arc discharge lamp comprising: an outer envelope; a generally tubular arc tube, having electrodes at each end, disposed within said envelope, said arc tube containing a fill including mercury, metal halide and an inert gas, the diameter of said arc tube being at a minimum at about the electrode regions thereof and gradually increasing therefrom to a maximum diameter at a region near the center of said arc tube; and a means to energize said arc tube. 